Adsorption of polyelectrolytes in the presence of varying dielectric discontinuity between solution and substrate

TL;DR

This study uses molecular dynamics simulations to explore how dielectric discontinuities and salt conditions influence polyelectrolyte adsorption and charge behavior near uncharged substrates, revealing conditions for monolayer formation.

Contribution

It provides new insights into the effects of dielectric contrast and salt type on polyelectrolyte adsorption and charge inversion, including conditions for monolayer formation.

Findings

PEs are repelled or attracted depending on substrate dielectric constant.

High salt and multivalent ions weaken PE adsorption.

Monolayer formation occurs only for weakly charged PEs at high salt.

Abstract

We examine the interactions between polyelectrolytes (PEs) and uncharged substrates at conditions corresponding to a dielectric discontinuity between the aqueous solution and the substrate. To this end, we vary the relevant system characteristics, in particular the substrate dielectric constant $\varepsilon_{\rm s}$ under different salt conditions. We employ coarse-grained molecular dynamics simulations with rodlike PEs in salt solutions with explicit ions and implicit water solvent with dielectric constant $\varepsilon_{\rm w} = 80$. As expected, at low salt concentrations, PEs are repelled from the substrates with $\varepsilon_{\rm s} < \varepsilon_{\rm w}$ but are attracted to substrates with a high dielectric constant due to image charges. This attraction considerably weakens for high salt and multivalent counterions due to enhanced screening. Further, for monovalent salt, screening enhances adsorption for weakly charged PEs, but weakens it for strongly charged ones. Multivalent counterions, on the other hand, have little effect on weakly charged PEs, but prevent adsorption of highly charged PEs, even at low salt concentrations. We also find that correlation-induced charge inversion of a PE is enhanced close to the low dielectric constant substrates, but suppressed when the dielectric constant is high. To explore the possibility of a PE monolayer formation, we examine the interaction of a pair of like-charged PEs aligned parallel to a high dielectric constant substrate with $\varepsilon_{\rm s} = 8000$. Our main conclusion is that monolayer formation is possible only for weakly charged PEs at high salt concentrations of both monovalent and multivalent counterions. Finally, we also consider the energetics of a PE approaching the substrate perpendicular to it, in analogy to polymer translocation.